1. Field of the Invention
The present invention relates generally to mechanical power management systems and specifically to rolling traction planetary drive systems.
2. Related Technology
Planetary gear systems have found significant use in many applications including automatic car transmissions and industrial equipment, as well as many others. Planetary gear systems are made of a central sun gear that rotates about a longitudinal axis and that is surrounded by one or more sets of planet gears. The planet gears in a plane surround the sun gear with each of their respective peripheral edges engaging the sun gear at its outer surface and. A ring gear surrounds the planet gears and engages the peripheral edge of each of the planet gears at their radially outermost point from the longitudinal axis. Each of the planet gears rotates about a planet shaft that forms its own axis. A planet carrier holds all of the planet shafts in their alignment and spatial distribution about the sun gear. The planet carrier is typically a disc or some other structure that is mounted coaxially about the longitudinal axis and can be capable of rotating about the longitudinal axis.
A planetary gear set can provide various levels of rotational speed reduction or increase and is very flexible. For instance, rotational torque input can be through any one or combination of the planet carrier, the ring gear or the sun gear and the output can be out of any one or combinations of these components as well. For example, for a high speed increase, an input can be provided to the planet carrier, the ring gear can be fixed so that it does not rotate, and the output can be taken out of the sun gear. The increase in speed, or transmission ratio, is a function of the ratio of the circumference of the ring gear to that of the planet gears; the planetary gear or “PG” ratio for this configuration. This means that if the ring gear circumference is four (4) times that of the planet gears, the sun gear will spin five (5) times as fast as the planet carrier. Therefore, a step-up of rotational speed of five times is achieved in such a configuration, or a reduction of five times is produced if the input is through the sun gear and the output is through the planet carrier.
However, the meshing of gear teeth in many existing planetary gear sets requires overcoming sliding friction that occurs as each gear tooth of one gear meshes with a corresponding tooth on another gear. The friction of this meshing is converted to heat, noise and deformation of the gears, and is therefore not transferred out of the gear set, resulting in a reduced efficiency of the gear set. This reduced efficiency is not satisfactory for many applications and an alternative type of reduction or step-up drive would be beneficial. While design alternatives to standard gear teeth exist that greatly improve the efficiency of such gear designs, such designs still do not provide a high efficiency at a low cost. Furthermore, it would be beneficial for a planetary gear set to achieve any or all of the following as well; a large speed change, the ability to produce any speed change ratio, the capacity for a very high rotational speed, a low manufacturing cost, long component life, flexible packaging for a wide variety of applications, or any combination of these. These and other advantages are achieved by some or all of the embodiments described herein.